Automatic volume control circuit



Maly 27, 1941- R L. |l|c 1 |NGswoR-rl-lv 2,243,423 I AUTOMATIC VOLUME CONTROL CIRCUIT Fil'ed May 9, 1939 AIV .f.

Patented May 27 1941 UNET sr'rss AUTOMATC VOLUME CONTROL CIRCUIT R. Lee Hollingsworth, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware My present invention relates to radio receivers vprovided with automatic volume control circuits,

and more particularly to a radio receiver having an automatic gain control circuit functioning to Vary the magnitude of signal coupling paths in the receiver. One of the main objects of my present invention is to provide a radio receiver of the type employing automatic gain control, wherein the gain control circuit is employed to vary the efficiency of transfer of signal energy between the signal collector and first tube of the receiver vand between amplifying stages within the receiver.

Another important object of my invention is to provide a radio receiver which utilizes an electronic capacitive reactance to transfer signal energy from a signal collector to the rst signal amplifier', and wherein a signal-derived voltage is utilized to vary the amplitude of the electronic reactance in the sense such as to provide a substantially uniform carrier amplitude at the receiver detector input circuit.

Another object of my invention may be stated to reside in the provision of electronic capacities between pairs of cascaded signal circuits of the receiver, and an automatic gain contro1 circuit of the receiver being utilized to vary the magnitudes of said capacitances and simultaneously to vary the gain of at least one signal amplifier tube of the receiver.

Still another object of this invention is to provide, in a radio receiver, an automatic gain control circuit which functions to vary the gain of signal amplifier tubes, and in addition is used to vary the amplitude of a coupling capacity between the antenna circuit and the input tube of the receiver, the capacity being provided by the electrodes of an electron discharge tube.

Still further objects of my present invention are' to improve generally the functioning of receivers employing automatic gain control, and more especially to provide a radio receiver, adapted to receive short waves, which has a highly reliable and efficient automatic gain control circuit embodied therein in an economical manner.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, wherein there is shown a circuit diagram of a superheterodyne receiver embodying my present invention, the receiving circuit is shown as comprising a signal collctor I which may be of any desired type. For example, the collector may be the usual antenna, or it may comprise a radio frequency distribution line, loop antenna, a dipole, or any other well known form of signal collector construction. 'lhe collector I is connected to ground through a path which includes an electron discharge tube 2, coil 3 and condenser B. 'Ihe cathode 5 of tube 2 is connected to the Vsignal collector I, while plate 6 is connected to one end of coil 3. A second path to ground includes the tunable signal circuit comprising coil I and variable condenser 8, the high potential side of circuit 1 8 being connected to the signal collector I. i lThe numeral 9 denotes an electron discharge tube arranged in the tunable radio frequency amplifier stage, the signal grid It of the tube 9 being connected through lead H to the high potential side of the tunable signal input circuit comprising coil I2 and variable condenser I3. The antenna circuit coil 3 is magnetically coupled to the input circuit coil I2 to provide signal energy transfer from the collector..I to the input circuit I2-I3. it being understood that the signal energy is transferred in that case by Virtue of the capacityl existing between cathode 5 and plate 6. This cathode to plate capacity is designated by I4, and is shown in dotted lines. The cathode of tube 9 is established at ground potential, while the plate I 4' thereof is connected to the common source of direct current potential I5 through a path which includes lead I6, coil Il and lead I8. Lead I3 is connected to the positive terminal of source I5. The screen grid electrode of tube 9 maybe connected to a desired positive potential point on source I 5.

The plate 6 of coupling tube 2 is connected to the positive terminal of source l5 so as to provide space current flow in tube 2, and such connectionv is made through a path which includes coil 3, the radio frequency choke I9, resistor 2d and lead 2|. 4The variable tuning condenser 22 is connected in shunt with coil Il, and circuit I'I-22 functions as an additional tunable signal circuit. In order to augment thetransfer of signal energy from the signal collector to the signal grid I0 of tube 9, there is provided a connection between the grid 5' of coupling tube 2 and the signal grid IIJ. This connection is provided when the switch 22 is adjusted in its uppermost position, that is to say in position to provide the short lead 23 in the connecting path between grid 5 and grid l5. In such position of the switch the signal energy is transferred to grid l through the grid to cathode capa/city of tube 2.

This grid to cathode capacity of tube 2 is designated by 24, and is denoted by the dotted line capacity on the drawing between cathode 5 and grid 5'. In order to augment the magnitude of capacity 24 there is provided a condenser 25 between the plate 5 and grid 5', and those skilled in the art are fully aware of the fact that such a plate to grid condenser augments the normal cathode to grid capacity value. Hence, it will be seen that when switch 22 is thrown into its uppermost position signal energy is impressed upon signal grid I0 thro-ugh two paths. One of these paths comprises the capacity i4 and magnetic coupling between coils 3 and i2, whereas theV other path comprises the capacity 24. It may be pointed out that the function of the tunable circuit 7-8 is to oifer an infinite impedance to the desired signal energy, and provide a low resistance direct current return path for the tube 2. In other words, inserting the tuned circuit 'l--8 in the antenna circuit provides an additional stage of selectivity ahead of the usual tunable input circuit |2|3 of amplifier 9.

When the switch 22 is adjusted to its lowermost position, that is when in contact with the ends of radio frequency choke coil 3i), the grids 5' and I0 are decoupled. In that case no signal transfer occurs through the capacity 24, and signal transfer is effective solely through capacity |4 and the magnetic coupling between coils 3 and |2. The radiofrequency amplifier tube 9 is coupled to the input electrodes of the first detector tube 3|. The cathode of the latter is established at ground potential, while the signal inputl grid 32 thereof is connected to the cathode 33 of a coupling tube 34. The plate 35 of :coupling tube 34 is connected to the lead I6 so that the plate 35 is established at a positive potential with respect to cathode 33. The capacity 36 existing between plate 35 and Cathode 32 providesmtge signal energy transfer path between the tunable signal circuit |'l-22 and the succeeding tunable signal circuit comprising coil 3l and variable tuning condenser 38. A

The high potential side of circuit 31-35 is connected to the junction of cathode 33 and grid 32, while the low potential side thereof is grounded. The tube 3| includes a plate electrode 4G and an oscillation electrode 4|. There is impressed upon electrode 4| Ilocally produced oscillations from any well known type of local oscillator 42, the local oscillations being transmitted to electrode 4| through a path which includes condenser 43 and lead 44. The lead is provided between the oscillator network 42 and the positive terminal of source I5 to provide a path through which the required positive potential can be ap plied to the oscillator tube of network 42. Numeral 45 denotes the oscillator tank circuit coil, while the variable tuning condenser of the tank circuit is denoted by 4'?. Numeral 48 denotes the positive screen grid electrode of tube 3|, and this screen electrode is positioned between sig` nal grid 32 and oscillation grid 4|. The electrode 48 may be connected through a voltage reducing resistor 43 to the positive potential lead I8.

It is not believed necessary to describe indetail the manner'in which the intermediate frequency energy is produced inthe plate circuit i potential.

of tube 3i. Those skilled in the art are fully acquainted with the fact that the signal and oscillator voltages modulate the electron stream flowing to plate 43. There is produced across the intermediate frequency output circuit 50 voltage of a predetermined operating intermediate frequency. The high potential side of the output circuit 55 is connected to plate 40, whereas the low potential end of the circuit is connected to an intermediate point on resistor 49 so that the plate 46 may be established at a desired positive The intermediate frequency voltage produced across circuit 50 is impressed upon the reso-nant input circuit 5| of the succeeding intermediate frequency ampliiier tube 52. Circuit 5| is tuned to the operating intermediate frequency, and is magnetically :coupled to circuit 53. The cathode of tube 52 may be grounded while the signal input grid thereof is connected to the high potential end of input circuit 5|.

The resonant output circuit 53, tuned to the operating intermediate frequency, is connected between the plate of amplifier 52 and the positive potential lead to resistor 49. The amplified intermediate frequency voltage developed across circuit 53 is impressed upon the second detector resonant input circuit 54, the latter being tuned to the operating intermediate frequency. The second detector is shown as embodying a tube of the double diode type, and this may be a tube of the 6H6 type if desired. The common cathode lead ofthe second detector tube 55 is grounded, while the diode anodes are connected to the opposite ends of the input circuit 54. The midpoint of the coil of circuit 54 is connected to ground through a path which includes the load resistor 56, and it will thus be seen that there is developed across resistor 56 a direct current voltage component, and an audio frequency voltage component assuming the received carrier is modulated by an audio signal. The tube 55 is embodied in a full wave rectifier circuit, and an intermediate frequency carrier by-pass condenser 5l is shunted across the load resistor 56.

The audio frequency voltage component developed across resistor 56 is impressed uponthe succeeding audio frequency amplifier through an audiofrequency transmission condenser 58, whereas the direct current voltage component is impressed `between the input electrodes of direct current amplifier tub-e 59. The cathode of tube 59 is established at ground potential, whereas the input grid thereof is connected to the anode end of lead resistor 56. The plate 60 of tube 53 is connected to the positive terminal of a direct current source Eil, and thenegative terminal of source 6l is connected to ground through a resistor 62.

In theY absence of received signal energy the rectified voltage developed across resistor 56 does not appear. According-ly the space current flow through tube 59 is maximum, andthe voltage drop across resistor 62 is also a maximum. A predetermined porticn of the direct current voltage developed across resistor 52Y is impressed upon the control grid of each of. tubes 2, 9, 34 and 52. The predetermined portion of voltage is chosen to have a magnitude, in the absence in received signals, such that the transmission eiiiciency of these controlled tubes is a. minimum. As a matter of fact, the minimum transmission efficiency is chosen so low that reproduction of noises, due to the low signal to noise ratio, is substantially prevented.- This. is accomplished by providingr a ladfIII which is connected to the desired' point on resistor 62 through a path including resistor 1I and theadjustable tap 12.

The signal input grid of amplifier' 52 is connected to lead 'I0 through a lead 14. The control grid of coupling tube 34 is connected to lead 'I0 by a lead 15, and the signal grid I of amplifier 9 is connected to lead 'IIJ yby virtue of its connection to lead II and coil I2. It is .pointed out that lead 'I0 is connected to the low potential end of input coil I2. It will now be seen that regardless of Whether the switch 22 is in its upper or lower positions, grid will be connected to lead I I and, therefore, to the biasing lead l0.

It will now be appreciated that in the absence of signal energy above a predetermined intensity magnitud-e, the control grids of each of tubes 2, 9, 34, 52 will have a biaswhich is determined by the position of tap I2 on resistor 62. 'Ihis bias will be su'iciently negative to prevent eiiicient transmission of electrical impulses through these tubes. In the case of coupling tubes 2 and 34 the consequence of such high negative biasing vwill be to diminish the magnitudes of capacities I4, or 24, and 36 to minimum values. It will, therefore, -be seen that there is provided a noise squelch circuit which functions electrically to prevent reproduction of undesirable noises when the received signal carrier amplitude is less than predetermined intensity.

Since the controlgrid of tube 59 is connected to the anode end of load resistor 56, it follows that as soon as the received signal energy increases in intensity the space current owof tube 5e will be reduced and the resulting direct current voltage drop across resistor 62 will also be reduced. Hence, as the received carrier amplitude in; creases, the noise reduction bias of the controlled tubes will bereduced, and maximum gain of these controlled tubes will be restored. To provide automatic gain control of the controlled tubes Vfor received signals whose carrier amplitude exceeds a predetermined amplitude, there is provided a lead 80 between the junction of lead and resistor 'II and the anode end of load resistor 56. The lead 80 includes the resistor 8l. The volume control network comprises resistors 8| and 1I and the condenser I3 which function as the usual time constant network. The resistors 8| and 'II also suppress pulsating components in the voltages applied to the control grids of the controlled tubes, and which voltages are derived from across resistors 56 and 62.

It will now be seen that when the anode end of resistor 56 becomes suinciently negative to cause the control grid of tube 59 to cut off the space current flow through the latter, then the direct current voltage across resistor 62 will be zero. The control grids of each of tubes 2, 9, 34 and 52 will now be electrically biased by virtue of the AVC path including leads 'IB and 80. As the received carrier amplitude increases, the direct current voltage developed across resistor 56 increases. As a result, the control grids of the controlled tubes will be biased in an increasingly negative manner to decrease the gain of each ofthe controlled tubes. Such gain decrease will occur to an extent sufficient to maintain the carrier amplitude at the detector input circuit 54 substantially uniform over a wide range of signal carrier amplitude variation at the collector I.

The rotors of the variable tuning condensers I3, 8, 22 and 38 may be uni-controlled with the rotors of the tank circuit tuning condenser 41,

and the dotted line representation denotes such commontuningmechanism. As those skilled in the art fully know, the frequency range of the tank circuit 46-41 will dier at all times from the signal frequency range bythe chosen intermediate frequency. While the present arrangement of Yautomatic gain control maybe utilized in the broadcast range of 50G` to 1500 kilocycles, it has particular value in the short wave frequency ranges because of the greater degree of automatic signalamplitude control provided by this system. It will be realized that in the present arrangement, and assuming that the tuning mechanism has been adjusted to receive a signal of relatively small amplitude as when receiving a distant station, .the automatic volume control (designated as AVC in the drawing) is at its minimum operation because a minimum direct current voltage is produced across resistor 56. In this case each of tubes 9 and 52 is operating at maximum gain, and each of coupling tubes 2 and 34 is operating with maximum gain.

Assuming, now, that switch 22 is in its upper position,.this means that signal energy is being transferred to the signal input circuit I2-I3 through'capacities I4 and 24, and that capacityr 36 is operating with maximum signal energy transfer between circuits II-22 and 3'I-38.v However, should the 'tuning mechanism be now adjusted to receive a strong local station, then the direct, current voltage developed across resistor 56 increases, and as a result the control grids of each of tubes 2 and 34 are biased to an extent such as greatly to decrease the magnitude of each of capacities I4, 24 and 35. At the same time, the gain of each of tubes 9 and 52 is decreased. The result of this reduction of the gain of each of the controlled tubes is greatly to decrease the overall signal transmission efficiency to the detector input circuit 54, and thus providing a much greater degree of control over the volume of the receiver. It will be understood that the reverse of the gain control sequence takes place should a strong carrier` amplitude begin to fadeso as to Vrequire greater amplification prior to the detector input circuit. The type of automatic volume control circuit shown herein, except for the action of the coupling tubes 2 and 34, is similar to that described in my U. S. Patent No. 2,144,221 granted Jan. 7, 1939.

While I have indicated and described a system for carrying my invention into eiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a signalamplier having a tunable signal input circuit, a signal collector, a coupling network between the collector and input circuit, said network comprising a tube having at least a cathode, grid and plate, said tube having its cathode to plate capacity connected between the collector and input circuit to provide a first signal coupling path, and means for varying the potential of the grid thereby" to adjust the Vmagnitude of said capacity, sai-d tube additionally having a cathode to grid capacity, and means connecting said last capacity between the collector and input circuit for providing a second coupling path between said collector and said input circuit.

2. In a receiver comprising a plurality of cascaded signal transmission tubes, a signal collector, an electron discharge tube having its plate to cathode capacity connected between the collector and the input electrodes of the rst of said tubes to provide a signal coupling path between the collector and the first transmission tube, said coupling tube having a control grid between the cathode and plate, and means for connecting the cathode to grid capacity thereof as a second coupling path between the collector and first transmission tube, means responsive to variations in the signal carrier amplitude for providing a variable unidirectional voltage, and means for controlling the space current flow between the said plate and cathode to adjust the magnitude of the said capacity.

3. In a radio receiving system, a signal co1- lector, a tunable signal amplifier, a coupling tube having a catho-de, grid and plate, means connecting the tube between the collector and amplier to provide the cathode to plate capacity as a signal coupling path therebetweema second signal amplifier, a second coupling tube provided with a cathode, grid and plate, means connecting the second coupling tube between said ampliers to provide the cathode to plate capacity of the second coupling tube as a signal coupling path between the ampliiiers, an automatic gain control circuit connected between the second amplifier output circuit and at least said iirst amplier, means for connecting the gri-d to cathode capacity of the first coupling tube as an auxiliary signal coupling path between the collector and the iirst ampliiier.

4. In combination, in a radio receiving system, a source oi signal oscillations, a tuned oscillatory circuit, a signal transmission path comprising a tube provided with at least a cathode, grid and plate, the plate to cathode capacity of the tube providing the said transmission path between the source and circuit, and means responsive to signal amplitude Variation for automatically varying the grid potential thereby to adjust the magnitude of said capacity, means for connecting the grid to cathode capacity of said l tube between the source and oscillatory circuit L" to provide ari additional signal coupling path, and said varying means functioning toy adjust the magnitude of the grid to cathode capacity.

5. In a signal receiver, a pair of cascaded similarly tuned. circuits, a tube provided with at least a cathode, grid and plate, means for connecting the tube between said circuits to provide the cathode to plate reactance of the tube asa signal transmission path between the circuits, and means responsive to an increase in the signal amplitude for increasing the negative potential of said grid to an extent suiilcient substantially to reduce the reactance value, and additional means for connecting the grid to cathode reactance of the tube between said circuits to provide an additional signal transmission path.

6. In a radio receiving system, a signal collector, a signal amplier having a tunedsignal input circuit, a tube provided with a cathode, grid and plate, means connecting the tube to said collector and input circuit to provide the grid to cathode capacity as a first signal transfer path, means including the cathode to plate capacity of said tube as a second signal transfer path, and signal-responsive automatic gain control means for adjusting the grid potential thereby to vary the value of said two capacities.

7. In combination with a signal responsive circuit followed by a signal utilization tube having at least a cathode, a control electrode and an output electrode, a signal voltage transmission path connected between said signal responsive circuit and a control electrode of said utilization tube, an electron discharge tube included in said path, and the capacity existing within said tube between the plate and cathode thereof providing a signal coupling element of said transmission path, a control grid locate-d between Vsaid vlast plate and cathode and providing capacity between the cathode and grid, said last capacity providing a second signal coupling element to said path, and means responsive to received signal energy above a predetermined amplitude for reducing the magnitude of sai-d capacities..

R LEE' HoLLINGswoRrH. 

